Most important to Milankovitch were the latitudes where the great ice sheets of the last ice ages formed. This graph shows how insolation has varied at 65 degrees N over the past 400,000 years.
While subsequent refinements in astronomy are reflected in this curve, it is quite similar to the one calculated by Milankovitch. By the 1920s, Milankovitch's theory of orbital forcing was practically complete, though some of his calculations would
take another two decades to finish. Once again, scientists used field studies to determine how well the theory of orbital forcing explained past climatic variations. Comparisons were initially positive. Sequences of glacial deposits in North America
and Europe seemed to support Milankovitch's theory. But these deposits were very difficult to date accurately, and so any correspondence between the timing of glaciation and Milankovitch's insolation curves remained speculative. Nevertheless, most
scientists supported the theory until the 1950s, when new developments in dating technology raised doubts about Milankovitch's ideas.
One of the unintended results of the atomic age was the discovery of radiocarbon dating methods. Over a
century after Louis Agassiz first proposed his glacial theory, geologists finally possessed a potent tool for determining the age of glacial deposits. They compared these dates with the insolation curves calculated by Milankovitch, and found that there
were more glacial advances in the last 80,000 years than Milankovitch's theory could account for. By 1965, the theory of orbital forcing lay once again in disrepute.
Thomas G. Andrews
NOAA Paleoclimatology Program
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